Modern vehicles are equipped with sophisticated electronic devices such as multimedia entertainment systems, GPS navigation systems, and the like. In such electronic devices, sound reproducing apparatuses such as compact disk players have been improved in performance, and there is an increasing demand for expansion of the sound range to reproduce very low frequency range sound with high power and high reproduction quality in the passenger compartment of a vehicle. Typically, such a low frequency range extends from about 25 Hz to about 100 Hz.
U.S. Pat. No. 4,924,962 discloses a sound reproducing apparatus for use in vehicle which includes at least one acoustic duct and a speaker unit disposed at a throat of the acoustic duct. An opening formed at other end of the acoustic duct faces the passenger compartment of the vehicle. The sound reproducing apparatus is designed to make the sound pressure frequency response curve relatively flat without increasing the level of distortion in the lower frequency range. Typically, the sound reproducing apparatus is mounted on the rear tray of the vehicle of a type so called a hatch-back car or station wagon which is constructed without any closed rear trunk.
Since the loudspeaker and the acoustic duct are mounted on the rear tray of the vehicle, an almost entire space in the rear tray is consumed by the sound reproducing apparatus, resulting in a poor space factor. Further, as will be explained later with reference to FIG. 2, when the sound source is provided at the rear tray or deck, the sound wave propagated to the front is reflected back to the rear because the material at the front wall (dash board, etc.) is hard, thereby causing a standing wave problem. Moreover, to achieve a sufficient sound volume for the front seat passenger, the rear seat passenger tends to be overwhelmed by the excessive sound volume.
U.S. Pat. No. 5,218,175 discloses a vehicle loudspeaker system which includes at least one elongated conduit having a proximal portion and a distal portion. A loudspeaker driver is disposed in a vehicle trunk and is attached to the proximal portion of the conduit. The loudspeaker driver has a vibration surface for generating sound pressure waves in response to electrical signals. The proximal portion of the elongated conduit is open to the passenger compartment. The elongated conduit has an effective length corresponding to a quarter wavelength of the lowest frequency of sound pressure waves. Typically, the proximal end of the conduit reach the front of the rear seat in the passenger compartment.
As will be explained later with reference to FIGS. 1 and 2, when the sound source is mounted under the rear seat, the vehicle loudspeaker system involves the standing wave problem and a rear reflection problem, although the degree of such problems is lower than that of the rear tray mounting as above. Further, since the proximal end of the elongated conduit reach the front of the rear seat, the sound quality will change depending upon the passenger's foot position or any articles placed on the floor.